1. Field of the Invention
This invention pertains to the dyeing of leather with azoic dyes and more particularly is directed to the azoic dyeing of dry or "crusted" leather, especially leather having a buffed suede surface, without wetting the leather with water.
2. Prior Art
The makers of leather goods face increasing need for leather of various colors and surface decoration effects in order to respond to rapid changes in the colors, textures, and decorative effects desired in high fashion clothing and accessories. They are also in need of economical processes to obtain such leather and to achieve uniform color and strength throughout. Processes by which dyes are chemically combined with leather and form uniformly within the structure and fibers of leather are most desired as they reduce frictional transfer of dye to other materials and changes in the color of leather upon abrasion. An acceptable process for dyeing and patterning dry leather should not make significant change in the esthetic or physical surface or bulk properties of the leather, such as results from the water-wetting of leather. Such processes are especially needed for dyeing and patterning dry napped suede leather for use in shoe tops, jackets, and other similar applications. Inexpensive patterning processes are also needed for preparing leather for use in handicraft kits.
The available art processes for dyeing dry leathers do not fill these needs. For dyeing such leathers in depth, it is the usual practice to rewet the leather with water containing additives which assist in removing oils and greases which would prevent even penetration of the dye, to adjust the acidity of the leather for optimum dye absorption and fixation, to introduce the dye into the leather, to adjust conditions to increase the efficiency of dye utilization, to rinse for removal of unabsorbed dye, to introduce fat liquor for desired flexibility and other physical properties, to redry the leather, to correct the effect of these processes on the surface and bulk esthetic and physical properties, and to continue with the conventional surface treatments which provide properties appropriate for the intended use. The dyes used have sometimes been azoic dyes formed in the presence of leather, (see U.S. Pat. No. 641,394, and Wilson, "Modern Practice in Leather Manufacture", Reinhold, New York (1941)), but usually have been preformed dyes of the acid, basic, direct, metallized, and mordant types, with dyes of different types frequently being used together or sequentially to obtain the desired degree of penetration and fullness of shade. Such operations are obviously complex, expensive, and difficult to control. Optical imaging of leather involving diazo dyes and aqueous solutions are disclosed in U.S. Pat. Nos. 1,472,323, 2,357,097 and 2,541,178.
Dry leathers have also been dyed by spraying or brushing with solutions of preformed dyes, for example in aqueous, alcoholic, or hydrocarbon solvents, (see "The Chemistry and Technology of Leather", Volume 3, Reinhold, New York (1956-1962)), but such processes usually result in incomplete and uneven penetration of the dyes.
The dyeing of buffed suede leathers has presented special problems because of the large amounts of dyes and time-consuming processes required to obtain complete and even penetration. In one process, for example, water-wet leather is impregnated with an aromatic amine with the assistance of an organic acid, the amine is diazotized with hydrochloric acid and sodium nitrite, the leather is thoroughly rinsed to remove unreacted sodium nitrite, the leather is impregnated with a material, such as toluene-diamine or a naphthol, which couples with the diazonium salt of the aromatic amine to form a dye within the leather, the leather is rinsed thoroughly under conditions which remove unreacted amines to avoid later darkening due to amine oxidation, and the leather is fat-liquored, dried, and finished in conventional fashion. In a similar process, the water-wet leather is impregnated with the coupling material under alkaline conditions, rinsed, acidified, impregnated with a diazonium component to produce an azoic dye within the leather, rinsed to remove unreacted diazonium salt, and finished in conventional fashion. After the drying of suede leathers dyed by either of these processes, they must be rebuffed to restore the suede surface, with a loss in surface depth of shade because of uneven penetration or formation of the dye in the leather fibers.
No satisfactory technique has been available for applying to leather large patterned areas of solid color or decorative patterns having fine detail. Techniques are particularly needed which are useful with napped suede leathers. Application of colored patterns to leather by block or letterpress printing without embossing the leather is virtually impossible, especially when the color is applied in large areas, in fine detail, or in separated portions of a large area. The stencil application of dye solutions is limited to the effects which can be obtained with such solutions. The silk screen application of pigmented pastes results in a stiffening of the surface which is particularly objectionable with buffed suede leathers. There is thus considerable need for a process which applies color to the surface of leather, and especially to napped suede leather, without significant change in the surface or bulk esthetic or physical properties of the leather. There are also needs for a process which applies one color to the surface and another color in depth throughout a piece of leather, and for a process which applies patterns to colored leather without expensive and complex equipment operated by highly skilled workmen.
It is an object of this invention to provide a simple and economical process for forming azoic dyes within the structure and fibers of leather without wetting the leather with water and without significant change in the surface and bulk esthetic or physical properties of the leather. It is another object of this invention to provide a process for the production of colored napped suede leather from dry napped leather without change in the surface properties of the leather such as would require rebuffing for restoration of the suede surface. It is a further object of this invention to provide a process for the production of dye patterns on the surface of leather, particularly while dyeing the bulk of the leather in the same or a different color in uniform strength and shade throughout and without significantly changing the surface esthetic or physical properties of the leather. These and other objects are achieved by the process of this invention as described hereinbelow.